Mathijs van Ledden, Maarten Kluijver (Haskoning Inc.), Bob Bass, Nancy Powell (USACE) and Bob Dean (University of Florida) HINDCAST OF WAVE OVERTOPPING AND LEVEE EROSION AT GRASS-COVERED LEVEES DURING KATRINA

Hurricane Protection System Details: Flood protection system with 350 miles of levees, floodwalls and structures Protecting 24% of the people of Louisiana with 460,000 households and 26,000 businesses (2000) Objective is a 1% protection level in 2011 (i.e. expected exceedance of 50% if you live 75 years in this area!)

Motivation of this work Current hydraulic design criteria based on current knowledge/guidance: 0.01 cfs per ft with 50% confidence 0.1 cfs per ft with 90% confidence Impact of this choice has significant costs/schedule impacts Strong need for more experimental validation, set-up of experiments underway Also: what are the lessons from Katrina?

Hurricane Katrina IPET investigated the causes and consequences of Katrina intensively Katrina was an intense and large storm Estimated return interval of the storm is around 400-years based on historical data

Levee breaches NO East

Levee breaches St Bernard

Approach Use the ADCIRC model for the surge levels Apply SWAN for the nearshore waves Determine overtopping rates based on empirical relationships Compare rates with observed damage

ADCIRC – peak of the storm Surge level modeling at Notre Dame University Katrina produced 16ft surge in the Lake Borgne area Source: Notre Dame University

ADCIRC – peak of the storm Source: Notre Dame University

SWAN grids 600 m / 200 m resolution Offshore waves from WAM model No bottom friction

SWAN results

Overtopping - assumptions Empirical overtopping formulations Van der Meer Pre-Katrina levee elevations Grass-covered levees Constant levee slope (1:5), no wave berm Perpendicular wave incidence (overtopping rates are conservative)

Overtopping results – St Bernard MRGO has severe overtopping rates 1 – 10 cfs/ft for hours Canaervon to Verret levee much lower (< 0.1 cfs/ft)

Overtopping results – NO East NO East back levee (~ 1 cfs/ft) Overtopping rates other sections are very low

Summary Overtopping rates and observations suggest an erosion/breaching threshold between 0.1 – 1cfs/ft Hindcast results do not disagree with the current design criteria but range is large

Suggestions for further work More detailed nearshore wave measurements (eg. Hurricane Gustav and Ike) Inclusion of wave direction in wave overtopping analysis Assessment of uncertainties in overtopping results Incorporate overtopping duration in criteria Lab/field experiments

Example of prototype experiments EU COMCOAST experiment

Conclusions Comparison between computed overtopping rates and levee observations during Katrina provide reasonable agreement with current hydraulic design criteria However: discrimination of levee sections/overtopping rates is not sufficiently detailed More experimental validation is necessary to confirm and refine design criterion

Thank you for your attention